Wireless communication systems, i.e. systems that provide communication services to wireless communication devices such as mobile phones, smartphones (often denoted by UE that is short for user equipment) as well as machine-type communication (MTC) devices, have evolved during the last decade into systems that must utilize the radio spectrum and other system resources in the most efficient manner possible. A reason for this is the ever increasing demand for high speed data communication capabilities in terms of, e.g., bitrate and to provide these capabilities at any given time, at any geographical location and also in scenarios where the wireless communication device is moving at a high speed, e.g., on board a high speed train.
To meet this demand, within the third generation partnership project (3GPP) work is being done regarding possible enhancements to radio resource management, RRM, performance in high speed train (HST) environments. The justification is that there are railways such as Japan Tohoku Shinkansen (running at 320 km/h), German ICE (330 km/h), AGV Italo (400 km/h), and Shanghai Maglev (430 km/h) at which vehicles travel at greater speed than 300 km/h and where there is demand for using mobile services.
For example, a new Remote Radio Head (RRH) arrangement for dedicated Single Frequency Network (SFN) High Speed Train (HST) scenario has been proposed in 3GPP; see e.g. 3GPP contribution R4-154518. This arrangement has shown to significantly improve the throughput for a wireless communication device traveling at speeds up to at least 600 km/h, by stabilizing the Doppler shift experienced by the wireless communication device (e.g. UE) and thus the Doppler shift experienced by a network node (e.g. a radio base station such as a eNodeB in a long term evolution (LTE) system) on the uplink. See for example 3GPP contribution R4-154520. Besides the stabilized frequency offsets experienced by wireless communication devices and network nodes with which wireless communication devices communicate via RRH's, it has also been shown that this RRH arrangement results in negligible inter-carrier interference (ICI) which results in a higher signal to interference ratio (SIR), and low impact of fading, all together leading to a higher carrier to interference and noise ratio (CINR) than otherwise possible. This in turn allows higher modulation orders and less robust encoding to be used, i.e. higher modulation and coding schemes (MCS) can be used. Hence, the system throughput is improved. A thorough analysis can be found in 3GPP contribution R4-154516.
A scenario as discussed above may comprise cells maintained by multiple RRHs along a railway track, with downlink transmission (DLTX) antennas/radio lobes and uplink reception (ULRX) antennas/radio lobes, respectively, pointing in the same direction. This is illustrated in FIG. 1, where a wireless communication device 101, e.g. an UE, is onboard a west moving high speed train 104 on a railway track 151. A group of wireless communication devices 161, e.g. a plurality of UEs and/or MTC devices, are also onboard the west moving high speed train 104 on the railway track 151.
Another group of wireless communication devices 163, similar to the group 161 in the form of, e.g., a plurality of UEs and/or MTC devices, are onboard an east moving high speed train 106 on a railway track 153. Yet another group of wireless communication devices 165 is located not on any of the trains 104, 106 but beside the tracks 151, 153. Wireless communication devices in this group 165 are moving at very low speeds (in comparison with the high speed at which the trains 104, 106 are moving) or they may even be stationary, e.g. due to the fact that users of wireless communication devices in the group 165 are waiting at a station for any of the trains 104, 106.
A first antenna node 110, which may be in the form of a RRH, maintains radio lobes including a transmission radio lobe 111, i.e. a downlink (DL) transmission (TX) DLTX lobe, and a reception radio lobe 112, i.e. an uplink (UL) reception (RX) ULRX lobe. Similarly, a second antenna node 120 maintains radio lobes including a transmission radio lobe 121, i.e. a DLTX lobe, and a reception radio lobe 122, i.e. an ULRX lobe. Similarly, a third antenna node 130 maintains radio lobes including a transmission radio lobe 131, i.e. a DLTX lobe, and a reception radio lobe 132, i.e. an ULRX lobe. As FIG. 1 illustrates, the transmission radio lobes 111, 121, 131 of the respective antenna nodes 110, 120, 130 are in this example all in one and a same direction, i.e. due west, and the reception radio lobes 112, 122, 132 of the respective antenna nodes 110, 120, 130 are also all in one and a same direction, i.e. due west. As introduced above, the unidirectional RRH arrangement for a SFN network in HST scenario illustrated in FIG. 1 may be such that multiple users are onboard each respective train 104, 106. All such user's UEs onboard the respective trains 104, 106 are experiencing and displaying the same Doppler shift characteristics.
There are drawbacks associated with prior art handling of radio frequency (RF) signals received in ULRX in an antenna node such as a RRH. Account has to be made for frequency offsets due to Doppler shifts on the ULRX for random access (RA) as well as for reception of, e.g., a physical uplink control channel (PUCCH) and/or a physical uplink shared channel (PUSCH).
For RA, frequency offsets may cause missed detection and false detections due to that the subcarrier spacing is smaller for the physical random access channel (PRACH) preamble than for other physical channels; 1.25 kHz versus 15 kHz. Hence, a frequency offset larger than ±0.5×1.25 kHz leads to that a PRACH preamble signal becomes shifted one or more subcarrier positions. This has a negative impact on the PRACH detection performance. For that reason, a restricted set of PRACH sequences has been introduced in prior art versions of the standard (3GPP TS 36.211 V8.9.0). The motivation has been that by narrowing down the PRACH sequences to look for, missed detections and false detections can be reduced. Recently it has been suggested to modify the set of allowed PRACH preambles even further (see 3GPP contribution R4-154364). This solution will only be available for new wireless communication devices supporting the new preamble set. The old wireless communication devices are unable to make use of this even further restricted set of PRACH sequences and, consequently, will still have problems to access the network, and will still cause problems for the network to accurately determine the correct identity of the wireless communication device carrying our random access.
For uplink physical control and data channels, an unaccounted frequency offset leads to leakage between subcarriers, i.e., inter-carrier interference, when the received symbols are demodulated. This degrades the sensitivity of the UL receiver and hence negatively impacts the system performance.
A network node that always searches over all possible PRACH sequence shifts and tries to estimate the frequency offset of each and every UE will have a significant workload and will additionally increase the rate of false PRACH detections.